Retarders are hydrodynamic brakes that utilize relative rotation of mechanical components within the hydraulic fluid present in the retarder to assist in slowing the vehicle in which the retarder is employed. Retarders are sometimes located in association with the output of the transmission and sometimes they are located in conjunction with the input to the transmission. Both locations have relative advantages and disadvantages, as is well known to this art. The present invention, however, is directed to the manner in which the operation of the retarder is controlled, and a mechanism embodying the concepts of the present invention is not dependent upon the relationship of the retarder to the transmission input or output.
Retarders have historically been employed in large vehicles to assist in vehicle braking or deceleration, particularly for the descent of steep hills in order to minimize extended usage of the vehicle brakes. The original retarders, or as they were then known, grade-retarding devices, were activated by moving the drive range selector lever to a "grade retarder range". In this position of the selector lever, all drive clutches were released, and to the surprise of the uninitiated driver, the grade retarder position of the selector lever would not afford any forward drive to the vehicle. It was truly a retard-only position.
Over the years, the operation and control of the retarder became quite sophisticated. The retarder itself typically constitutes a bladed rotor wheel that is operatively connected to the rotatable member for which rotation is to be selectively "retarded", and the rotor wheel is disposed adjacent to a bladed stator that is fixedly positioned. In the vehicular art, when a component is thus fixedly positioned, it is said to be "grounded" or "fixed to ground". The rotor and the stator are contained in, and generally conform to, a toroidal working chamber that is selectively filled with hydraulic fluid. The hydraulic interaction between the rotor and the stator effects the hydrodynamic braking action. As such, controlling the volume and pressure of the hydraulic fluid within the working chamber serves to regulate the operation of the retarder. Typically, the retarder is controlled by a series of valves that are opened and closed selectively to determine the flow of hydraulic fluid entering and exiting the toroidal working chamber.
In the current state of the art, automatic transmissions will only passively assist the function of the retarder when certain conditions are met. In particular, a dump valve may be hydraulically connected to the clutch housing so that when an excessive speed is reached, the pressure within the clutch housing will be relieved by releasing the oil contained therein through the dump valve to a sump or other hydraulic return system. Moreover, there is no prior art arrangement by which hydraulically to limit horsepower absorption in input retarders.